Getting infected with a hookworm doesn’t sound like something you should do on purpose, but if patients can stomach the idea, their health could very well benefit.

In the first clinical trial of its kind, a team led by scientists from James Cook University in Australia inoculated volunteers with human hookworm larvae (Necator americanus) to see if these parasites can improve their metabolic health.

The two-year study included 24 participants, each of whom showed heightened insulin resistance at the start of the trial.

By the end of the investigation, researchers found those who were infected with hookworms showed a significant reduction in their insulin resistance scores compared to those who were given a placebo.

That’s not enough to recommend the practice just yet, but the findings join those of another pioneering study published in 2021 that infected participants with hookworms and found it benefited their gut’s microbiome.

The researchers say their trial provides “proof of principle that hookworm treatment may stabilize or improve key determinants of metabolic health such as insulin resistance.”

Parasitic worms, like hookworms, have been living inside humans through much of our species’ history, and while some infections can put our health at risk, others barely make their presence known.

They might even bring some good. In recent decades, scientists have noticed that in places where parasitic worms are endemic, people report fewer metabolic and inflammatory diseases.

What’s more, recent studies have found that when people are ‘dewormed’, their insulin resistance suddenly increases. This is a factor that helps regulate blood sugar levels, and it is a risk for the development of metabolic issues like type II diabetes and cardiovascular disease.

The current trial at James Cook University is one of the first in the world to purposefully infect participants with hookworms for a limited period of time.

Initially it included 40 adult volunteers, all of whom had relatively high insulin resistance scores. The 24 adults who completed the trial were inoculated with either a placebo or 20 or 40 infectious larvae of human hookworm. Neither the patients nor the researchers were aware of what treatment was given to who.

Mild to moderate gut issues, like bloating, nausea, vomiting, constipation, or diarrhea, were common among those with the hookworm infection, but the symptoms were manageable. Only three people dropped out because of discomfort.

After a year of regular physical and mental health check-ups, the group of participants that were infected with hookworms ended up scoring significantly lower on an assessment for insulin resistance called HOMA-IR.

Those who received 20 hookworm larvae saw their scores drop from 3.0 units to just 1.8 units.

Those who were infected with even more larvae only saw their scores drop from 2.4 to 2.0.

Meanwhile, the placebo group saw a median increase in their scores of 0.8 units.

“These lowered HOMA-IR values indicated that people were experiencing considerable improvements in insulin sensitivity – results that were both clinically and statistically significant,” explains bioscientist and lead author Doris Pierce.

But there’s still a lot of unknowns to tease apart. For instance, how do hookworms impact human metabolism? And why doesn’t a higher dose of hookworm improve insulin resistance in a linear way?

The sample size of this first trial is simply too small to answer those questions.

The team is now gearing up to organize an international, large-scale study to investigate further.

The study was published in Nature Communications.